The invention relates to a magnetic recording and reproducing method and apparatus adapted for magnetic disk drives used with personal computers, large computers, communications servers, and so on.
Magnetic disk units, particularly hard disk drives (HDDs) using rigid magnetic disks, have features of large storage capacity, fast accessibility, and high data transfer rates and have been widely used as peripheral storage units of information processing units such as personal computers, large computers, communications servers, etc. As the MPU operations speed increases and communications infrastructure improves and expands, the demand for increased storage capacity of the HDDs will increase.
In order to increase the storage capacity of the HDDs, it is required to increase the surface recording density. Constant endeavors have been made to increase the recording density from various aspects of technologies, such as signal processing technologies typified by PRML (partial response maximum likelihood), high-accuracy head positioning technologies, tri-bology technologies important to interface between head and disk, head technologies typified by giant magnetoresistance (GMR) heads, and magnetic disk medium technologies. The size of recording cells formed on a recording medium is reduced as the recording density increases, which is accompanied by a reduction in leakage signal magnetic fields from the medium that contribute to the reproduction of signals. Therefore, in order to ensure a high signal-to-noise ratio (SNR) of reproduced signals, it is required to reduce noise associated with the medium.
Conventional magnetic disks use a thin film made of an aggregate of Co-based magnetic crystal particles for their recording surface. With such a thin film consisting of magnetic particles, magnetic transition regions are easy to fluctuate due to exchange interaction of the magnetic crystal particles. In order to reduce medium noise, therefore, it is required Ito separate the particles from one another with a nonmagnetic material. That is, to reduce noise, the volume particle content must be lowered, which results in a further reduction in signal field. Thus, the effect of improving the signal-to-noise ratio by noise reduction can be little expected.
Even in the case where the exchange interaction of magnetic particles is completely broken, the fluctuation of magnetic transition corresponding to the particle size is inevitable in principle. In order to further reduce medium noise, it is needed to reduce the size of magnetic particles. The smaller the magnetic particles, the smaller the magnetic energy of the particles. For example, the particle size required to make a high-density recording of the 10 Gb/in.sup.2 class will be at most 10 nm. At this level of particle size, a low magnetic energy of the magnetic particles causes a problem of magnetic stability against thermal agitation at room temperature.
The way to increase the magnetic energy of the magnetic particles with their size kept small is to increase their magnetic anisotropy energy. Excessively high magnetic anisotropy energy will make the recording saturation magnetic field required with reversal of magnetization so large as to exceed the ability of a recording head. While an attempt is being made to use a soft magnetic material for magnetic poles of a recording head, it cannot be expected to improve the head recording capability as long as the recording track width is made narrower as the recording density increases.
Thus, with the conventional magnetic recording technique that uses a magnetic recording medium having a recording layer consisting of magnetic particles, in order to reduce medium noise sufficiently for increased recording density, it is required to reduce the size of magnetic particles constituting the recording layer. In order to overcome thermal agitation with the particle size kept small, it is needed to make large the magnetic anisotropy energy of the magnetic particles. In obtaining a sufficient overwrite erase ratio with a head adapted for narrow tracks, there is a drawback that the upper limit of the magnetic anisotropy energy is limited, thus the increase of recording density has been limited.
In general, a system expected as the magnetic recording technique that overcomes the limitations of recording density is vertical magnetic recording. In the vertical magnetic recording, as with the longitudinal magnetic recording, the medium recording layer is composed of magnetic crystal particles. The vertical magnetic recording is distinct from the longitudinal magnetic recording in that the recording layer has the axis of easy magnetization in a direction perpendicular to its surface. Unlike the longitudinal magnetic recording, the vertical magnetic recording has a feature that, even if the size of recording cells is small, the state of magnetization will not change under the influence of a demagnetizing field and hence the recording layer can, in principle, be increased in thickness. It is thought that there is the possibility that all the conditions of low noise, high thermal agitation resistance, and high recording sensitivity may be satisfied, provided that the thickness of the recording layer can be increased. That is, even if the size of particles is made small, the thermal agitation resistance can be increased with increasing layer thickness and a material having excessive magnetic anisotropy energy need not necessarily be used.
In the vertical magnetic recording as well, it is needed that the recording layer must be reduced, to some degree, in thickness because of requirement of uniformly magnetizing the recording layer for recording in the direction of its thickness and applying as abrupt magnetic fields as possible to the recording layer. This is inconsistent with the above-described feature of the vertical magnetic recording. A method has been proposed which, through the use of a two-layer recording medium having a soft magnetic layer as the underlying layer, allows abrupt recording magnetic fields to be applied to the recording layer to thereby produce uniform recording in the direction of its thickness even if it is thick. However, if a soft magnetic film is formed over a large-area surface such as a magnetic recording medium, random magnetic domain walls become easy to be formed, independently of recording patterns. And the domain walls change their position with ease, resulting in the occurrence of many burst-like errors.
As described above, the conventional magnetic recording techniques using an aggregate of magnetic particles as the recording layer of a magnetic recording medium are encountering the limits of improvement in the recording density under the circumstances that:
(1) the size of the magnetic particles constituting the recording layer has to be made small in order to reduce medium noise down to a sufficient level; As described above, the conventional magnetic recording techniques using an aggregate of magnetic particles as the recording layer of a magnetic recording medium are encountering the limits of improvement in the recording density under the circumstances that: PA1 (2) the size of the magnetic particles has to be made small in order to reduce medium noise down to a sufficient level; PA1 (3) even if the particle size is reduced, the magnetic anisotropy energy of the magnetic particles has to be made large in order to overcome thermal agitation; and PA1 (4) trying to obtain a sufficient overwrite erase ratio with a narrow-track head results in that the upper limit of the magnetic anisotropy energy is limited.
The vertical magnetic recording, which in principle allows higher recording densities than the longitudinal magnetic recording, has the possibility that all the conditions of low medium noise, high thermal agitation resistance, and high recording sensitivity may be satisfied if the recording layer thickness is reduced. Further, its requirements of making uniform recordings in the direction of thickness of the recording layer and applying abrupt recording magnetic fields to the recording layer result in a basic problem that it is inadvisable to make the recording layer thickness large. Moreover, when a two-layer medium having a soft magnetic film as its underlying layer is used, random magnetic domain walls become easy to be formed, independently of recording patterns, and the domain walls change their position with ease, resulting in a problem of the occurrence of many burst-like errors.